The invention relates to a method for blow molding containers, in which, after thermal conditioning, a preform of a thermoplastic material is deformed into a container within a blow mold by the influence of blowing pressure along a transport path in the area of a heating section, and wherein at least one parameter characterizing the blow molding process is measured and supplied to a control device, which acts on at least one adjusting element for changing at least one of the parameters which influence the blow molding process.
The invention moreover relates to a device for blow molding containers of a thermoplastic material, which includes at least one heating section arranged along a transport path of a preform and at least one blow molding station provided with a blow mold, and wherein a control device is used, and has at least one sensor for measuring at least one parameter which characterizes the blow molding process, wherein the sensor is connected to the control device.
In the formation of containers by the influence of blowing pressure, preforms of a thermoplastic material, for example, preforms of PET (polyethylene terephthalate), are supplied within a blow molding machine to different processing stations. Such a blow molding machine typically has a heating device as well as a blowing device in whose area the preform which has previously been thermally conditioned is expanded into a container biaxial orientation. The expansion takes place by means of compressed air which is introduced into the preform to be expanded. The technical procedure in such an expansion of the preform is explained in DE-OS 43 40 291. The introduction of the pressurized gas, mentioned above, also includes the introduction of compressed gas into the developing container bubble, as well as the introduction of compressed gas into the preform at the beginning of the blow molding process.
The basic construction of a blow molding station for deforming containers is described in DE-OS 42 12 583. Possibilities for thermally conditioning the preforms are explained in DE-OS 23 52 926.
Within the device for blow molding, the preforms, as well as the blow molded containers, can be transported by means of various manipulating devices. Particularly useful has been found the use of transport mandrels onto which the preforms are placed. However, the preforms can also be manipulated by means of other support devices. The use of gripping tongs for manipulating preforms and the use of expanding mandrels, which for support can be introduced into an opening area of the preform, also belong to the available constructions.
A manipulation of containers with the use of transfer wheels is described, for example, in DE-OS 199 06 438 in an arrangement of the transfer wheel between a blow molding wheel and a deliver section.
The manipulation of preforms already mentioned above takes place, on the one hand, in the so-called two-stage method, in which the preforms are initially manufactured by a blow molding process, are subsequently intermediately stored, and are only conditioned later with respect to their temperature and blown into a container. On the other hand, the use of the so-called single-stage methods in which the preforms are, immediately following their production by means of injection molding and a sufficient solidification, suitably thermally conditioned and subsequently blown up.
With respect to the blow molding stations used, various embodiments are known in the art. In blow molding stations which are mounted on rotating transport wheels, a book-like opening of the mold carriers can be frequently found. However, it is also possible to use mold carriers which are slidable relative to each other, or are guided in some other manner. In stationary blow molding stations, that are particularly suitable for accommodating several cavities for the formation of containers, typically plates that extend parallel to each other are used as mold carriers.
Prior to carrying out heating, the preforms are typically placed on transport mandrels which transport the preform either through the entire blow molding machine, or which rotate only in the area of the heating device. In stationary heating for the preforms, such that the openings of the preforms are oriented downwardly in the vertical direction, the preforms are usually placed on a sleeve-shaped holding element of the transport mandrel. In suspended heating for the preforms, in which the preforms are oriented with their openings upwardly in the vertical direction, usually spreading mandrels are inserted into the openings of the preforms which clamp the preforms.
When carrying out a container formation by utilizing blow molding technology, a significant object is to achieve a predetermined material distribution in the container wall. A significant parameter for predetermining the resulting material distribution is the heat distribution realized prior to blow molding in the preforms.
The heat distribution is typically realized in such a way that, in a circumferential direction of the preforms, an equal temperature level is produced, and a temperature profile is produced in a longitudinal direction of the preforms. Moreover, also through the wall of the preform, a suitable temperature profile from the outside to the inside is predetermined. Basically, it can be assumed that areas of the preform with a lower temperature lead to thicker wall areas of the blown container, and that the warmer areas of the preform are stretched to a greater extent when carrying out the blow molding deformation and, as a result, lead to thinner wall areas of the blow molded container.
The temperature in the area of the preforms can be measured with so-called pyrometers. A measurement of an actual wall thickness in the area of the blown containers can be effected by means of so-called wall thickness sensors which operate for example optically, or with the use of sound waves.
Further significant parameters for influencing the material distribution in the blow molded container are the stretching speed, the relationship with respect to time of the stretching process relative to the blow gas supply, the volumetric flow of the blow molding gas, as well as the pressure pattern over time in the expansion of the preforms into containers. In particular, the control of the actual blowing pressure has been found to be difficult because, between a control valve for presetting the blowing pressure and the preform to be expanded, there is a flow path with different cross-sectional sizes of the lines and throttles which influence the flow and, moreover, the volume increase of the preforms during the deformation into the container causes a return reaction to the pressure which is generated. On the other hand, moving the stretching rod into the preform results in a reduction of the available volume. Moreover, there are relatively complex interactions between individual parameters which influence the concrete actual material distribution in the blow molded container.
The large number of parameters and the interactions between the parameters have the result that, instead of an actual regulation, frequently only a control takes place with the consideration of empirically determined and manually predetermined adjustments. Actually realized regulations typically refer to individual parameters, without sufficiently taking into consideration the complexity of the blow molding process.
In typical controls, an adjustment to influencing parameters, which are unknown or changeable with respect to time, is effected in that an operator of the blow molding machine examines the produced bottles, and manually changes parameters of the blow molding process through the use of a control panel. Such a procedure has the result that, initially a plurality of bottles are produced whose properties deviate from the predetermined desired values and, partially iteratively an approximation to the ideal bottle configuration takes place. Even if properties of the blow molded bottles are measured and there is a back coupling within the scope of regulations, initially the regulation deviations must occur in order to be able to react to them. Moreover, the existing regulation concepts have, because of the complex interactions between the individual blow molding parameters, weaknesses occur which lead to regulations of deviations which are not satisfactory or result in long regulation times.